Field of the Invention
The present invention generally relates to the measurement of air speed in aircraft and, more particularly, to an improved indicated air speed (IAS) measuring instrument capable of measuring very low air speeds.
Background Description
Measurement of air speed in aircraft is usually accomplished by placing a pitot-static tube (i.e., pitot tube) in the airstream for which the speed is to be measured. The ram effect of air entering the pitot tube creates a positive air pressure in the tube proportional to air speed. It is this ram air pressure which is measured to determine air speed.
For nearly one hundred years, general aviation aircraft have used a bellows actuated instrument to measure the difference in pressure between the pitot tube and a reference static tube to provide a measure of air speed. A bellows type air speed indicator is illustrated in FIG. 1. The pitot tube 10 is mounted on an exterior surface of the aircraft and pneumatically connected by way of conduit 11 to a diaphragm 12. A static port 13 is similarly pneumatically connected by way of conduit 14 to the diaphragm 12. The diaphragm 12 reacts to a differential pressure produced by the ram air from the pitot tube 10 and the static air from the static port 13 to produce a movement of the diaphragm 12 indicated by the double arrow 15. This movement is mechanically coupled to the gear 16 which drives the pointer 17 of an indicator dial 18 which is provided with indices indicating air speed in knots.
FIG. 2 shows a plot of ram pressure versus airspeed which shows the very low ram air pressures associated with low indicated air speeds. It will be observed that the plot is an exponential function. Because ram pressure is an exponential function, the pressure below about 20 knots is so low that the pneumatic-mechanical (bellows) system is unable to measure air speed accurately. This is due to stiction, friction, and mechanical advantage of the gear train, all in the air speed indicator instrument. Accordingly, most air speed indicators do not register below about 20 knots. For fixed wing aircraft, this shortcoming does not have a dramatic impact since the majority of a flight for a fixed wing aircraft flight is not at low air speeds. Rotary wing aircraft, however, are an entirely different matter. The failure of known air speed indicators to register below about 20 knots is especially disadvantageous for helicopters because at and below translational air speed, lift and helicopter performance begin to change dramatically.
In recent years, some pneumatic-mechanical measurement devices have been replaced by electronic transducers which convert ram air pressure to electronic values which can be converted to airspeed. FIG. 3 is a block diagram of an uncompensated electronic indicated air speed (IAS) instrument. As in the air speed indicator shown in FIG. 1, there is a pitot tube 30 and a static port 31. The pitot tube 30 is pneumatically connected by a conduit 32 to the positive input of an electronic pressure transducer 33, while the static port 31 is pneumatically connected by a conduit 34 to the negative input of pressure transducer 33. The pressure transducer 33 produces an output which is an electronic representation of the differential pressure input between the pitot tube 30 and the static port 31. If the output is an analog output, an analog-to-digital (A/D) converter (not shown) would be provided to convert the analog signal to a digital value. For the purposes of this illustration, it is assumed that the A/D function is incorporated into the pressure transducer 33. This digital value is input to a processor 35 which generates, either by computation or by table lookup, an output to a digital display 36.
While the electronic IAS instrument of FIG. 3 represents a distinct improvement over the instrument shown in FIG. 1, these devices have operational limits due to output drifts with, for example, time and transducer temperature. Curve 40 in FIG. 4 shows a time plot of uncompensated indicated air speed measurements at various temperatures. The temperature changes over time are shown by curve 41. Actual air speed in this case is about five knots. Curve 40 shows a typical drift of an uncompensated electronic IAS instrument measurement. Large changes in measured air speed occur for small changes in temperature at actual air speeds below 20 knots for uncompensated methods.
EP 0 188 909 A2 by Atkinson (“Atkison”) discloses a system for compensating for sensor calibration drift for low airspeed applications. Atkinson teaches a pre-flight procedure whereby the same pressure is supplied to both a total pressure sensor and a static pressure sensor. If there is a difference in the readings between the two sensors, this constitutes a calibration error. The value of the error is saved into memory and subtracted from the differential readings during regular operation of the sensor during flight. Atkinson is limited by the fact that calibration error is only assessed pre-flight and treated as a static constant. In reality, calibration error of an IAS instrument fluctuates with changes in, for example, temperature, time (e.g., different times during the same flight), and altitude.
CN201402160Y discloses a pitot tube flowmeter with a zero point calibration function. During ordinary operation of the flowmeter, a total pressure tube and a static tube are both connected to a differential pressure sensor. A separate valve is provided for each tube to selectively close off the total pressure tube or the static tube from the sensor. To perform the zero calibration function, the two valves are closed and a third valve is opened which connects the two ports to the flowmeter to one another. The output of the transducer with the inputs cross connected is treated as a drift value and is recorded. Returning to regular operation, the recorded drift value is subtracted from the difference between the total pressure tube and static tube. This system is inefficient with the necessity for three separate valves. It further introduces an additional error source by measuring the error signal at a pressure which is not equal to local atmospheric pressure.
When there are rapid changes in temperature around an IAS instrument, such as, for example, during start-up, during descent from higher altitudes to lower altitudes or vice versa, when cabin heat settings are changed, or when the flight takes the aircraft from cloudy to clear conditions, the pressure transducer needs to be rezeroed much more often to maintain acceptable low speed measurement accuracy. This can be a problem because rezeroing requires a few seconds to perform and during that time, air speed measurement is unavailable.